1. Field of the Invention
This invention relates to high temperature heating and more particularly it concerns novel and improved electrical resistance heating arrangements characterized by low cost and reliable long duration operation.
2. Description of the Prior Art
The present invention is especially useful in connection with ladle preheating in metal foundries. In foundry operations molten metal is carried in a bucket-like ceramic lined ladle and is poured from this ladle into molds where it cools and solidifies to a particular configuration. Before a ladle can receive molten metal, however, it must first be preheated to a temperature of for example 1200.degree. C, otherwise its ceramic lining will crack as a result of thermal stress when molten metal at e.g. 1400.degree.-1600.degree. C is poured into it. Even the preheating of the ladle must not be done too rapidly or the thermal stresses encountered in the preheating operation itself will cause the ladle lining to crack. In general, several hours are required for bringing a ladle up to temperature in a safe manner.
In the past, foundry ladles were preheated by special gas or oil fired burners whose flame was directed down into the center of the ladle. The hot flaming gases from the burner would blow back up along the inner sidewalls of the ladle to heat them; and then the gases would escape out through the top opening of the ladle. This technique of directing a burner flame down into a ladle had the advantage of providing an even distribution of applied heat to the ladle walls but it was wasteful of energy since hot gases were continuously emitted out from inside the ladle. This excessive use of heat energy was especially aggravated due to the fact that it is usually necessary to maintain at least one spare preheated ladle at operating temperature at all times during foundry operations.
Electrical resistance type heaters have previously been employed for melting of metal or for maintaining molten metal in its liquid state. Examples of such heaters are seen in U.S Pat. No. 3,737,153 and in German Pat. No. 965,835. These electrical resistance heaters, which operate by directing a high amperage electrical current through a graphite element, have not been suitable for ladle preheating because the graphite element would deteriorate relatively rapidly at the high temperatures and oxidizing atomspheres encountered in the preheating operation.